I have built a simple amplifier circuit using a EM11G LM 386N-1 amplifier chip which follows the following design (cap and resister values are slightly different):

EM11G LM 386N-1 circuit

I connected this mic to the circuit, and the circuit produce some good sound in a pair of headphones:

VideoSecu High Sensitive Preamp Mini Audio Pickup Tiny Spy Microphone with Power Bypass for Security Audio Sound Voice Monitoring Recording and bonus Warning Decal WD4

After this mic was damaged, I tried to replace it with this mic:

Sound Tech CM-1000 Table Top Conference Meeting Microphone with Omni-Directional Stereo 3.5mm Plug

I connect it so that the left and right channel are connected to the 'input' and the ground ring is connect to the common ground.

enter image description here

The new mic does not work in this configuration. The sound that should be picked up by this mic is not coming out of the headphones, not even slightly. If I listen closely, I think I can hear a faint radio station, but that's it. I know that the mic is working because I plugged it into a voice recorded and tested it. No problems.

Is there something that the voice recorded is doing differently to cause the mic to work? How do I need to change the circuit so that the mic will be heard?

Edit: Would adding a preamp like this (between the mic and 386N) possibly correct the problem?

enter image description here


1 Answer 1


The first mic appears to have an integrated preamp, which presumably amplifies the microphone level signal (very low amplitude) quite a bit, giving you something closer to a line signal, which then drives the 386.

As for the second mic, note that one of the reviews seems to indicate that the microphone is an active microphone (and therefore requires a power source) but does not come with a power source. It is expected to be connected to a device that provides power. It might need a few volts, it might need full phantom power, or it might need some bizarre arbitrary voltage supply.

Alternatively, you might have shorted it or otherwise miswired it, because there is nothing that says a microphone will have the same connection scheme as a pair of headphones. Unfortunately, there is no standard for wiring a microphone to a 3.5mm jack (e.g. stereo, tip/ring bridged, DC supply on ring, balanced, and so on).

Edit: even if the second microphone doesn't require power, you need a mic preamp stage. The output of a typical passive microphone is ~50dB below line level. Your 386 circuit is entirely insufficient for amplifying that small of a signal; you will need to build a mix preamp.

In short, there are many things that could be going wrong...

  • \$\begingroup\$ Somehow the voice recorder can sort it out. Do those tend to supply power to their mike jacks? I should try putting a volt meter to the jack to find out... Thanks for pointing out that the mic jack may not be wired like the headphone jack. \$\endgroup\$
    – Hoytman
    Commented Jan 23, 2016 at 4:39
  • \$\begingroup\$ Haven't a clue without more information about the voice recorder. Also, see my edit. Even if it is a passive mic, your circuit won't work. \$\endgroup\$
    – uint128_t
    Commented Jan 23, 2016 at 4:45
  • \$\begingroup\$ Would adding a preamp like this (see question edit) possibly correct the problem? \$\endgroup\$
    – Hoytman
    Commented Jan 23, 2016 at 4:51
  • \$\begingroup\$ If it's an electret microphone, yes. But it might not be an electret microphone. \$\endgroup\$
    – uint128_t
    Commented Jan 23, 2016 at 4:53
  • \$\begingroup\$ Would it not work it the mic was not electret, or could it cause damage? If this is the case, then what else could I try? \$\endgroup\$
    – Hoytman
    Commented Jan 23, 2016 at 4:55

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